Connectors or contacts used in electrical circuits must possess excellent resistance to oxidation and corrosion in addition to good electrical conductivity. Electrodeposits of gold on a suitable metallic substrate such as copper or copper alloys have commonly been used but more recently it has been proposed to use palladium to replace gold in view of its comparable electrical conductivity and oxidation/corrosion resistance and because it is relatively inexpensive. Other requirements of the deposited metal are firstly that it must have very low and preferably zero transverse porosity at economical thicknesses in order that problems associated with corrosion of the substrate are obviated, secondly that the metal is ductile so that it can withstand flexing in service without exposing the substrate and thirdly that the metal is wear-resistant so that repeated mating of the connectors does not expose the substrate.
A currently available palladium bath, which is described in U.S. Pat. No. 3,920,526, is being proposed for uses such as coating of connectors and comprises specified quantities of the compound Pd(NH.sub.3).sub.2 Cl.sub.2 and ammonium chloride and sufficient aqueous ammonia to provide a pH of at least 8.8. However, it is common for both engaging parts of the connectors to be plated with the palladium and in such cases it has been found that the resistance to sliding wear is insufficient. A similar bath is reported in a book entitled "Precious Metal Plating" by Fischer and Weimer, published by Robert Draper Limited, Teddington, England. Fischer and Weimer also report on electrodepositing palladium from an acid bath prepared from palladium chloride, sodium nitrite, boric acid and sodium chloride. The nitrite bath contains the anionic complex Pd(NO.sub.2).sub.4.sup.2-. However, it has been found that palladium deposits from such baths have high transverse porosity and are therefore unsuitable as coatings for electrical contacts or connectors.
There is therefore a need for a method of producing palladium electrodeposits that possess adequate sliding wear resistance and adequately low transverse porosity in addition to possessing all the other properties referred to above.
We have now surprisingly found that this can generally be achieved by means of a duplex palladium electrodeposit formed from two specifically defined plating baths and that this duplex deposit retains its economical advantages over other non-duplex deposits because, surprisingly, the necessary thickness of each layer required in the duplex coating is generally considerably less than the minimum thickness necessary to obtain a desired property of the respective single layer coatings.
Hereinafter the term electrical contact material refers to electrical contacts, connectors, switches, printed circuit plates and the like or any part of such device.